In a motor-driven power steering control apparatus that provides assist power generated by an electric motor in accordance with steering torque applied to a handle by a car driver, assist torque approximately proportional to steering torque is determined, and a torque proportional gain, which keeps such a proportional relationship, is set large so that steering force of a driver is reduced and smoothed, and besides, vibration such as torque ripple produced by a motor or disturbance transmitted from a road surface is suppressed, improving feeling of the driver.
In the motor-driven power steering control apparatus of this type proposed in the past, a mixed filter including a low-pass filter (LPF) and a high-pass filter (HPF) is used to suppress the vibration such as motor torque ripple or road surface disturbance, a steering component is extracted by the LPF to control steering assist torque, and a high-frequency vibration component such as torque ripple is extracted by the HPF, and the high-frequency vibration component is controlled by a different controller (different gain) from a controller for the low-frequency vibration component (for example, refer to patent document 1).
Moreover, a motor-driven power steering control apparatus is previously proposed, in which a bandpass filter (BPF) is used to extract a vibration component of motor rotation speed or angular velocity of a steering angle, and a signal as a product of the vibration component multiplied by a gain is subtracted from a target current so that a new target current is set, thereby negative feedback control of angular velocity is established to reduce vibration (for example, refer to patent document 2).
Besides, there exist a motor-driven power steering control apparatus having hysteresis processing means that filters a steering torque signal by using a hysteresis function in order to remove small variation such as bit variation from the steering torque signal (for example, refer to patent document 3).                Patent document 1: Japanese Patent No. 2838053        Patent document 2: JP-A-2003-26022        Patent document 3: JP-A-2004-170174        